Cast single crystal articles such as turbine blades and vanes can be produced by several techniques. A common method involves the use of a starter zone at the bottom of the mold wherein a plurality of columnar grains are formed. A "nonlinear" or transversely displaced crystal selector (e.g., a helix-shaped passage) connects the starter zone to the article cavity, and this selector insures that only one columnar grain grows into the article cavity. Single crystal castings also can be produced using molds which have a vertical "slender projection" at the bottom of the article cavity (i.e., a linear or non-transversely displaced "neck") as described in Bridgman U.S. Pat. No. 1,793,672.
When traditional directionally solidified (columnar-grained polycrystalline) articles are desired, the starter zone communicates directly with the article cavity (no crystal selector is present) as described in Chandley U.S. Pat. No. 3,248,764, VerSnyder U.S. Pat. No. 3,260,505, and Piearcey U.S. Pat. No. 3,494,709.
These techniques generally are restricted to producing articles that have the "natural" crystal growth directional (e.g., the &lt;001&gt; direction in face-centered cubic and body centered cubic metals) oriented along the "longitudinal" dimension of the article. This longitudinal dimension generally is normal to the chill plate and/or parallel with the direction of heat withdrawal. In addition, with these methods of making a single crystal, it can be difficult or impossible to simultaneously align the secondary orientation of the grain relative to a desired "transverse" dimension of the article i.e., to orient an orthogonal &lt;010&gt; or &lt;100&gt; direction within the article cavity).
These limitations can be avoided by using seed crystals as described in the aforementioned Bridgman patent. Briefly stated, one of Bridgman's methods involves use of a mold with a cavity that terminates with a vertical passageway, the end of which constitutes a mold aperture. Seed crystals of any desired primary and/or secondary orientation are inserted into the aperture, liquid metal is formed in (or preferably poured into) the mold, and solidification proceeds by epitaxial growth from the seed (in the presence of a longitudinal temperature gradient) using practices which avoid the nucleation of new grains.
It is well known to those skilled in the art that effective use of the Bridgman seeding methods requires that the size and shape of the mold aperture closely approximate the cross section of the seed crystal, both to preclude metal running past the seed and out of the mold, and to avoid the nucleation of new grains in interstices between the mold and the seed. In addition, it also is well known that it is generally desirable for both technical and economic reasons to use seeds of relatively small crosssectional area. These considerations can restrict the utility of the Bridgman seeding method in the following ways:
(1) It can be difficult or impossible to accommodate individual deviations in the longitudinal crystallographic orientation of seeds relative to their external envelopes since they must mate with a fixed mold aperture. PA1 (2) It can be awkward to position small diameter seeds (e.g., 0.030") in the proper secondary orientation, as a result of ordinary handling and manipulation problems. PA1 (3) When ceramic molding techniques are utilized, as is preferred in the production of directionally solidified turbine blades and vanes, the dimensional reproducibility limitations of current ceramic molding methods can limit the accuracy of seed crystal positioning. This is of particular concern when precise orientation relationships are required in the cast article. PA1 (4) Also with respect to ceramic molds, it is difficult to reuse seeds, since after shell removal and cutoff, the seeds must be sorted, cleaned, usually reinspected for grain orientation, and then repositioned within another cluster. PA1 (5) The use of mold passages and apertures that are small relative to the size of the article cavity can present structural rigidity problems during pattern assembly. Ancillary members (e.g., ceramic tie bars) may be needed to support the pattern, which adds cost and weight to the assembly, and may under certain circumstances, compromise technical effectiveness during solidification, such as by altering heat flow characteristics or by inducing the undesirable nucleation of crystals at points of contact with the article cavity. PA1 (6) Small mold passageways and apertures can also present difficulties during pattern removal (e.g., dewaxing). Pattern materials usually expand during heating (e.g., steam dewaxing or "burnout") and it is advantageous to have more than one relatively large mold opening. Although many molds can be dewaxed successfully through the top (via the metal feed), the presence of a large aperture at the bottom of the mold increases the speed and effectiveness of the operation, while minimizing the probability of shell damage. PA1 (7) Small mold passageways and apertures can restrict the cross sectional area of metal which conducts heat to the chill plate. This limitation obviously can exist with small nonlinear passageways, and Erickson, et al. U.S. Pat. No. 3,724,531
teaches the use of a double-wall mold construction method to ameliorate that difficulty.
It will be obvious to those skilled in the art that many of these restrictions become more onerous when multiple cavity molds are involved or when more than one seed is used with an article cavity.